1. Field of the Invention
The present invention relates to a solar battery cell. More particularly, the present invention relates to a solar battery cell having a plurality of power-generating regions connected in series, each power-generating region having an insulative transparent substrate, a front electrode, an intermediate transparent conductive film, a photoelectric conversion layer formed of stacked semiconductor films, and a back electrode.
Further, the present invention relates to a manufacturing method of the solar battery cell as described above. More particularly, the present invention relates to a manufacturing method of the solar battery cell having a plurality of power-generating regions connected in series, each power-generating region having an insulative transparent substrate, a front electrode, an intermediate transparent conductive film, a photoelectric conversion layer formed of stacked semiconductor films, and a back electrode.
2. Description of the Background Art
A photovoltaic power generating system using solar battery cells to directly generate electric energy from sunbeam has recently undergone rapid technical development to the extent permitting practical use. Now, the photovoltaic power generating system is considered a promising clean energy technique which can protect the global environment of the 21st century from environmental contamination due to combustion of fossil fuels.
Here, materials for solar battery cells used in the solar battery are largely classified into the following four groups:    (i) IV group semiconductors;    (ii) compound semiconductors (III-V group, II-VI group, I-III-VI group);    (iii) organic semiconductors; and    (iv) compounds, such as TiO2, used for wet type photovoltaic power generation.
Among them, practical utilization of the IV group semiconductors has proceeded most vigorously because of their lower manufacturing cost compared to the other materials. The IV group semiconductors are largely divided into (i) crystalline based semiconductors and (ii) non-crystalline semiconductors (also called amorphous semiconductors).
Materials of the crystalline based semiconductors used for the solar battery cells include monocrystal silicon, monocrystal germanium, polycrystal silicon, and microcrystal silicon.
The non-crystalline semiconductors used for the solar battery cells include amorphous silicon.
Here, the solar battery cells made of such semiconductor materials are largely classified into the following three types:    (i) pn junction type;    (ii) pin junction type; and    (iii) hetero junction type.
Among them, the pn junction type is often used in a solar battery cell employing a crystalline based semiconductor in which the carrier diffusion distance is great. By comparison, in a solar battery cell employing a non-crystalline semiconductor in which the carrier diffusion distance is small and local levels exist, the pin junction type is often used, since it is advantageous to make the carriers move by drift with an internal electric field in the i (intrinsic) layer.
Generally, a solar battery cell of the pin junction type has a typical structure where a transparent conductive film of SnO2, ITO, ZnO or the like is formed on an insulative transparent substrate of glass or the like, a photoelectric conversion layer is formed with p layer, i layer and n layer of non-crystalline semiconductors stacked thereon in this order, and a back electrode of metal thin film or the like is stacked further thereon. There also is a solar battery cell of the pin junction type having a reversed structure where a photoelectric conversion layer is formed with n layer, i layer and p layer of non-crystalline semiconductors stacked in this order on a back electrode of metal thin film or the like, and a transparent conductive film is stacked thereon.
Currently, the method of stacking p-i-n layers in this order has become dominant, because of the reasons that the transparent insulative substrate can also serve as a surface cover glass of the solar battery cell, and that, with development of a plasma-resistant transparent conductive film such as SnO2, it has become possible to use plasma CVD to form the photoelectric conversion layer of non-crystalline semiconductors thereon.
Here, the energy conversion efficiency of the solar battery manufactured with the above-described solar battery cells is represented in % as a ratio between solar radiation light energy being input and electric energy being output from a terminal of the solar battery.
That is, the conversion efficiency η is defined as follows. η=(electric output of the solar battery)/(solar energy input to the solar battery)×100%
To standardize criteria for measurement of the conversion efficiency, the International Electrotechnical Commission has defined a nominal efficiency as a ratio, in percentage, of the maximal electric output with respect to the input light power of 100 mW/cm2 when the load condition is changed, with an air mass condition of the solar radiation being AM 1.5.
Nominal efficiency ηn of a solar battery can be derived from maximal output point voltage Vmax, maximal output point current Imax, open voltage Voc and short-circuited photoelectric current density Isc that are obtained from measurement of an output of the solar battery based on the above-described conditions.
Namely, when the input light power of the sunbeam is represented as Pin and the effective light-receptive area of the solar battery is represented as S (cm2), nominal efficiency ηn can be expressed as follows:                               η          n                =                ⁢                                            (                                                V                  max                                ·                                  I                  max                                            )                        /                          (                                                P                  in                                ·                S                            )                                ×          100          ⁢          %                                        =                ⁢                                            (                                                V                  oc                                ·                                  I                  sc                                ·                FF                            )                        /            100                    ⁢                                           ⁢                      (                          mW              ·                              cm                                  -                  2                                                      )                    ×          100          ⁢          %                                        =                ⁢                              V            oc                    ⁢                                           ⁢                                    (              V              )                        ·                          I              sc                                ⁢                                           ⁢                      (                          mA              ·                              cm                                  -                  2                                                      )                    ⁢          FF          ⁢                                           ⁢                      (            %            )                                                            where          ⁢                                           ⁢          FF                =                ⁢                              (                                          V                max                            ·                              I                max                                      )                    /                                    (                                                V                  oc                                ·                                  I                  sc                                            )                        .                              
The FF represents a fill factor, which constitutes an important index to performance of the solar battery.
As seen from the above expression, in the measurement with the input power regulated to 100 mW/cm−2, if Voc and Isc and hence FF are found through the experiment, a product thereof can be calculated to obtain the nominal efficiency of the solar battery.
The nominal efficiency of the solar battery obtained in this manner is generally from about 10% to about 20%. In other words, from about 80% to about 90% of the solar energy is not converted to electric energy but dissipated somewhere, possibly due to the following reasons.
(i) Cell surface reflects sunlight.
For example, a silicon solar battery has a silicon plate surface like a dark mirror, since the cell surface reflects the light. The cell surface is normally coated with an anti-reflection film to reduce such reflection.
(ii) It is impossible to absorb the sunlight entirely in full wavelengths.
The sunbeam mostly has the wavelengths of 0.2-3 μm (ultraviolet light, visible light, infrared light), which cannot be absorbed entirely. Which wavelength is more efficiently converted to electric energy depends on how and of which material the solar battery is made.
(iii) Free electrons and free holes are not generated 100%.
The free electrons and free holes may not be generated upon irradiation of light. Even though generated, they have certain lifetimes and may be lost at once, before being taken out as electric energy.
(iv) Yield of free electrons and free holes is not 100%.
A certain amount of the free electrons and free holes generated at and separated from a pn junction surface or the like are lost during migration toward the electrodes.
(v) Solar battery has resistance therein.
Since the silicon material, electrode portion and others have electric resistances, electricity generated cannot be completely taken to the outside, leading to reduction of FF.
Based on the foregoing, eliminating or alleviating these causes will lead to further improvement of the nominal efficiency of the currently available solar battery.
In addition to such efforts, it has also been attempted to improve the nominal efficiency of the solar battery by increasing the degree of integration thereof, specifically by providing a larger number of photoelectric conversion layers on the substrate of the solar battery.
For example, in a solar battery cell called a multiple junction type, a large number of photoelectric conversion layers are stacked one another in a travelling direction of light, such that the respective photoelectric conversion layers absorb the light sequentially from the short-wavelength side, to generate voltages corresponding to their respective bandgaps Eg.
It is often the case that such a structure is employed together with a method for improving the conversion efficiency of the solar battery, where a material of high reflectivity, such as Ag, Al or the like, is used for the back electrode of the solar battery cell to increase the light absorbing efficiency in the photoelectric conversion layer, and a transparent electrode is inserted between the photoelectric conversion layer and the back electrode to scatter light, thereby increasing the optical path length of the incident light in the photoelectric conversion layer. In this case, a material containing SnO2, ZnO, ITO or the like is often used for the transparent electrode.
In recent years, a solar battery cell having a power-generating region where two or three layers of photoelectric conversion layers are stacked one another for the purpose of further increasing the voltage generated in one power-generating region, has been developed vigorously. Also under development is a solar battery cell having a multi-bandgap type power-generating region where an upper photoelectric conversion layer and a lower photoelectric conversion layer have different bandgaps from each other to effectively utilize the energy of different wavelengths of the sunbeam.
In general, in the case where electronic equipment is driven by a solar battery or the solar battery is used for power supply, it is necessary to employ a solar battery cell of a large area having a plurality of power-generating regions connected in series, since the voltage generated in one power-generating region is not more than 1 V.
For example, a common solar battery cell is formed on an insulative substrate using a patterning process or the like. In this case, a plurality of power-generating regions, each having a transparent electrode, a photoelectric conversion layer and a back electrode, are formed on a transparent insulative substrate such as a single glass substrate, and the neighboring power-generating regions are connected in series.
In the patterning process of the solar battery cell described above, the patterning has conventionally been performed by etching using a resin mask or the like. With this method, however, a large number of processes are required for formation of the stacked structure. In addition, it can handle only the substrates of a limited dimensional range, and the effective area of the power-generating regions within the substrate of the solar battery would be small. Pinholes would be generated in the photoelectric conversion layer with such a wet process, and patterning would be difficult in the case of a substrate having a curved surface.
On the other hand, patterning utilizing heating by laser irradiation has recently been developed and come into wide use. This patterning utilizing laser can reduce the number of processes for forming the stacked structure. It also makes it possible to form a solar battery cell on a substrate of large area, or even on a substrate of an arbitrary structure such as one having a curved surface. The effective area of the power-generating regions within the substrate of the solar battery also increases. This patterning is suitable for continuous production and automated production.
In particular, while the conventional patterning by etching using a resin mask or the like would require a distance of about 3 mm between the neighboring power-generating regions, the patterning utilizing heating by laser irradiation can reduce it to not more than 0.5 mm. Accordingly, adopting such patterning utilizing heating by laser irradiation can increase the effective area of the power-generating regions within the substrate of the solar battery, and improve the output of the solar battery cell per unit area by not less than 30%.
In the solar battery cell having a power-generating region in which photoelectric conversion layers are stacked as described above (herein, also referred to as the “stacked type solar battery cell”), when the power-generating region has a two-layer structure of an upper photoelectric conversion layer (herein, also referred to as the “upper cell”) and a lower photoelectric conversion layer (herein, also referred to as the “lower cell”), the power-generating efficiency of the relevant stacked type solar battery cell usually becomes maximum when the current values of the upper cell and the lower cell are equal to each other.
Here, assume that photovoltaic power generation is conducted using a solar battery cell provided with a power-generating region having a stacked body of amorphous silicon thin films as an upper cell and a stacked body of microcrystal silicon thin films as a lower cell. In this case, it is conceivable, for the purpose of improving the power-generating efficiency of the solar battery cell, to increase the film thickness of the upper cell and hence the current value being generated therein, so as to balance the current values in the upper and lower cells.
When the film thickness of the upper cell of the solar battery cell is increased, however, although the current values generated in the upper and lower cells may be balanced, such an increased film thickness of the upper cell would cause further degradation of light, thereby adversely decreasing the power-generating efficiency of the solar battery cell.
A conceivable way to overcome the above-described problem is to insert a transparent conductive film (herein, also referred to as the “intermediate transparent conductive film”), having a refractive index of light different from those of the upper and lower cells, between the upper cell and the lower cell of the solar battery cell. Reflecting light with the intermediate transparent conductive film will allow even a thin upper cell to generate a high current value.
Here, such a stacked type solar battery cell will be able to generate a sufficient amount of voltage only if it has a stacked structure (herein, also referred to as the “serial stacked structure”) where a plurality of power-generating regions, each having stacked photoelectric conversion layers, are connected in series. In the case of a stacked type solar battery cell having a transparent conductive film interposed between upper and lower cells, however, forming such a serial stacked structure by conventionally known patterning utilizing heating by laser irradiation (herein, also referred to as “laser patterning”), for example, would cause short circuit in the upper cell, leading to degradation of the output of the stacked type solar battery cell.
In other words, in order to form a serial stacked structure in a solar battery cell, it is necessary to connect a back electrode and a front electrode in the neighboring power-generating regions with a contact line filled with a conductive material so as to connect the power-generating regions in series. With the solar battery cell having the intermediate transparent conductive film formed between the upper and lower cells, if such a contact line is formed by a conventional method like laser patterning, it is highly possible that the contact line will directly contact the intermediate transparent conductive film.
For example, FIG. 5 shows a structure obtained when a serial stacked type solar battery having power-generating regions of a two-layer structure of upper and lower cells is formed with a common serial stacked structure as in the one unprovided with an intermediate transparent conductive film 1. In this serial stacked structure, however, the intermediate transparent conductive film 1 is formed between an upper cell 2 and a lower cell 3. As such, when a contact line open groove 9 for electrically connecting a back electrode 6 and a front electrode 5 is formed to connect the neighboring power-generating regions, intermediate transparent conductive film 1 and contact line open groove 9 will directly contact with each other. This renders upper cell 2 short-circuited, thereby posing a problem that the output of the relevant serial stacked type solar battery cell becomes less than half the intended amount.
Currently, strenuous efforts for research and development have been made in a variety of fields to solve the above-described problem. For example, Japanese Patent Laying-Open No. 9-129903 discloses a serial stacked structure as shown in FIG. 6. This serial stacked structure, however, has the following disadvantages. Assume that the front electrode 5, the upper cell 2 and the intermediate transparent conductive film 1 are being subjected to laser patterning at the same time. In this case, if processing of front electrode 5 is insufficient, although front electrodes 5 in the neighboring power-generating regions may be separated from each other, sublimated front electrode 5 would be redeposited on the side surfaces of upper cells 2. This may cause short circuit between intermediate transparent conductive film 1 and front electrode 5 even if ultrasonic cleaning or the like is conducted, thereby degrading the output of the solar battery cell. This leads to reduction of the manufacturing yield of the solar battery cells.
Further, in the solar battery cell having the serial stacked structure as shown in FIG. 6, when lower cell 3 is being formed by CVD using a material containing microcrystal silicon, lower cell 3 is required to have a film thickness of not less than 2 μm. In addition, since there is a site where lower cell 3 and insulative transparent substrate 4 directly contact with each other, peeling of lower cell 3 is very likely to occur during formation of lower cell 3, depending on the deposition condition thereof. This leads to deterioration of reliability, reduction of the manufacturing yield of the solar battery cells, and restricted improvement in efficiency of the cells.
In general, laser scribing of front electrodes 5 is followed by measurement of separation resistance between the cells. If the separation is insufficient, the succeeding step is prevented from starting or a portion failed in processing is repaired so as to effectively use the material or improve an average output. Such confirmation and repair, however, are impossible during the processes of the structure shown in FIG. 6.
Further, Japanese Patent Laying-Open No. 9-129906 discloses a serial stacked structure as shown in FIG. 7. In a solar battery cell having the serial stacked structure of FIG. 7, again, the lower cell 3 needs to have a thickness of not less than 2 μm when it is formed by CVD using a material containing microcrystal silicon. In addition, there is a site where lower cell 3 and insulative transparent substrate 4 directly contact with each other. Accordingly, it is very likely that lower cell 3 is peeled off during its formation, depending on the deposition condition thereof. This results in poor reliability and a decreased manufacturing yield-of the solar battery cells, thereby hindering improvement in efficiency of the cells.
As described above, in forming a solar battery cell having a serial stacked structure with an intermediate transparent conductive film formed between the upper and lower cells, a specific structure needs to be adopted to prevent short circuit between the back electrode and the intermediate transparent conductive film. To date, any structure conventionally known has not solved the problems of deterioration in reliability of the solar battery cell and a decrease of the manufacturing yield of the solar battery cells.